Since the discovery of biologically active metabolites of vitamin D there has been much interest in the preparation of structural analogs of these metabolites, because such compounds may represent useful therapeutic agents for the treatment of diseases resulting from calcium metabolism disorders. A variety of vitamin D-like compounds have been synthesized. See, for example, U.S. Pat. Nos. 3,741,996 directed to 1.alpha.-hydroxycholecalciferol; 3,907,843 directed to 22-dehydro-25-hydroxycholecalciferol; 3,906,014 directed to 3-deoxy-1.alpha.-hydroxycholecalciferol; and 4,069,321 directed to the preparation of various side chain-fluorinated vitamin D.sub.3 derivatives and side chain-fluorinated vitamin D.sub.3 derivatives and side chain-fluorinated dihydrotachysterol analogs.
Certain fluoro derivatives of the accepted hormonal form of the vitamin, 1,25-dihydroxycholecalciferol (1,25-(OH).sub.2 D.sub.3) are of particular interest because they are characterized by at least as great if not greater activity than 1,25-(OH).sub.2 D.sub.3. These are, 24,24-difluoro-1,25-(OH).sub.2 D.sub.3 (U.S. Pat. No. 4,201,881) and 26,26,26,27,27,27-hexafluoro-1.alpha.,25-dihydroxycholecalciferol (U.S. Pat. No. 4,358,406).
Also of interest is the 26,26,26,27,27,27-hexafluoro derivative of 25-hydroxycholecalciferol (see U.S. Pat. No. 4,248,791) which exhibits greater vitamin D-like activity than 25-hydroxycholecalciferol.